Flexible, low density thermoplastic foams and methods for lowering the density and increasing the flexibility of thermoplastic foams

ABSTRACT

A flexible, low density thermoplastic foam and a method for lowering the density and increasing the flexibility of a thermoplastic foam having a melting temperature and being either amorphous with a softening temperature or semicrystalline with a glass transition temperature. The method comprises the steps of (a) decreasing the pressure on the thermoplastic foam to a subatmospheric pressure, further providing that while the thermoplastic foam is under the subatmospheric pressure, the thermoplastic foam is also at a temperature in the range of less than the melting temperature and greater than the softening temperature if the thermoplastic foam is amorphous, or greater than the glass transition temperature if the thermoplastic foam is semicrystalline, whereby the thermoplastic foam expands; (b) then exposing the thermoplastic foam to a superatmospheric pressure and a secondary expansion gas for a sufficient amount of time to allow the secondary blowing gas to permeate into the thermoplastic foam; and (c) then releasing the superatmospheric pressure on the thermoplastic foam whereby the thermoplastic foam expands. With this method, it is possible to produce thermoplastic foams having densities as low as 0.008 grams/cc. Also included in this invention are insulations made from these low density foams.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a division of co-pending U.S. patentapplication Ser. No. 09/236,745, filed Jan. 25, 1999.

FIELD OF THE INVENTION

This invention relates generally to thermoplastic foams and theprocessing of thermoplastic foams, and more particularly to a method forlowering the density and increasing the flexibility of thermoplasticfoams.

BACKGROUND OF THE INVENTION

Thermoplastic materials are those that soften and flow upon applicationof pressure and heat. Thermoplastic foams are defined, generally, asfoams made from thermoplastic resins. Because thermoplastic materialsregain their original properties upon cooling, most thermoplasticmaterials can be remolded many times. Examples of thermoplastic resinsinclude poly(vinyl chloride), polyethylene polystyrene, acrylate resins,and poly(ethylene terephthalate).

It is known to produce thermoplastic resin foam materials havingsubstantially closed-cell structures by intimately incorporating withinthe resin material a volatile organic liquid which vaporizes uponheating to form a gas (the liquid is known as the blowing agent, itsresultant vapor the blowing gas). It is also known to use a solidsubstance as the blowing agent where the solid substance decomposes toform the blowing gas. The vapor created from the blowing agent is theblowing gas (often also referred to as simply the blowing agent) andcauses the thermoplastic to expand and form a cellular mass.

Thermoplastic resin materials that have been foamed by the action of avolatile organic blowing agent producing a primary foaming gas maythereafter be induced to further expand. This secondary expansion isachieved by exposing the foamed material to another gas (a secondarygas), such as steam or air, which has a permeability rate greater thanthe permeability rate of the primary foaming gas through the cell wallsof the foamed mass. During the exposure to this secondary gas, thematerial is reheated to a heat softening temperature. The secondary gas,which has a permeability rate through the cell wall greater than that ofthe primary gas already in the cell, permeates the cell wall and joinsthe primary gas inside the cell. At the heat softening temperature, thecombined effect of the primary gas and the secondary gas causes furtherexpansion of the initially foamed material. The result is a lowerdensity foam product.

It is further known that thermoplastic resin materials that have beenfoamed by the gas emitted upon decomposition of a solid substance maythereafter be induced to further expand. This further expansion isachieved by heating the foamed material to a temperature near themelting point of the resin while subjecting it to a secondary gas atsuperatmospheric pressure. After this step is performed, the foamedmaterial is reheated to a heat softening temperature at a lower pressure(i.e. atmospheric pressure). This causes the gas to expand inside thecells. The combined expansion of the primary gas and the secondary gas(which has entered the cells of the foamed material primarily because ofthe internal/external pressure differential during the application ofthe superatmospheric pressure) produces a lower density foam product.

SUMMARY OF THE INVENTION

The present invention is an improved, low density thermoplastic foam andan improved method for treating thermoplastic resins to achieve foams oflower density and increased flexibility. Foams having densities as lowas 0.008 grams/cubic centimeter (g/cc) are obtainable.

The invention involves a multi-step process. The first step calls fordecreasing the pressure on a primarily foamed thermoplastic resin and,while the foam is subject to this decreased pressure (under at least apartial vacuum), increasing the temperature of the foam. The foamtemperature is increased to a point between the glass transitiontemperature and about the melting point of the foam, if the resin ismade from a semicrystalline resin. If the resin is amorphous, thetemperature is raised to a point between the softening temperature andthe melting point of the amorphous foam. While these temperature andpressure conditions are sustained, the foam expands. The foam expansionis the result of the cells in the foam undergoing an increase in volumedue to the temperature increase and pressure decrease. The order inwhich the temperature and pressure are changed is irrelevant—they mayeven be adjusted simultaneously. Furthermore, if the foam is takendirectly from a foam extrusion process, the foam may already be at theproper temperature. In fact, from the foam extrusion process, thetemperature may even have to be lowered before allowing the first stepto occur. The conditions reached in the first step are held for apredetermined time, to allow adequate foam expansion, before moving tothe second step.

The second step involves exposing the primarily foamed (and expanded)thermoplastic resin foam to a secondary expansion gas for a sufficientamount of time to cause secondary expansion. Secondary expansion occurswhen the secondary gas permeates the cells of the thermoplastic resinand joins the primary blowing agent inside each cell. The pressure underwhich the foam is subject during this second step is preferably at leastabout 1 pascal (Pa) above atmospheric pressure. For a faster permeationrate and subsequent expansion, the pressure should be at least about 500kilopascals (kPa) above atmospheric pressure. When the pressure isreleased, the foam will expand again, thereby lowering its density.

In this second step, it is preferred that the temperature is in the samerange as that used in the first step. Although it is possible to performthe second step at temperatures up to the melting point and down toambient temperatures, the process would, in the later case, beunsuitably long. The temperature can be maintained from the first step,or the material can be cooled and later reheated. In addition, thetemperature can be increased before, during, or after the pressure isadjusted. After the gas permeates the foam cells during the second step,the pressure is released and expansion occurs. The foam can be cooledbefore, during, or after the pressure is released. Preferably, the foamwill be cooled after pressure is released and expansion is allowed tooccur.

The density reduction achieved by this process can be up to 96%;controlled density reductions can be in the range from about 15% toabout 96%. More preferred density reductions will be in the range offrom about 50% to about 96%.

BRIEF DESCRIPTION OF THE DRAWING

The invention is best understood from the following detailed descriptionwhen read in connection with the following drawing, in which

FIG. 1 is a block diagram that schematically illustrates a continuousoperation for carrying out the steps according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an improved, low density thermoplastic foam andan improved method for treating thermoplastic foams to achieve foams oflower density and increased flexibility. The invention involves atwo-step process. The first step involves increasing the temperature ofa primarily foamed thermoplastic resin to a first temperature which isbetween the glass transition temperature of the foamed thermoplasticresin and the melting temperature of the foamed thermoplastic resin ifthe resin is semicrystalline, and between the softening temperature andmelting temperature if the resin is amorphous. While this firsttemperature is maintained, the pressure acting on the primarily foamedthermoplastic resin is reduced to a subatmospheric pressure. The orderin which the temperature and pressure are changed is irrelevant, andthey may be adjusted simultaneously. The conditions reached in the firststep are then held for a predetermined time before moving to the secondstep.

The second step involves exposing the primarily foamed thermoplasticresin to a secondary expansion gas. During this exposure, the pressureis increased to a superatmospheric pressure, and the foam temperaturecan go from ambient up to the melting point. Although the hightemperatures do not have to be used in this step, the cooler the foam iskept, the slower the permeation rate of gas into the cells. Thisexposure to elevated temperature and pressure is allowed to occur for asufficient amount of time to cause the secondary expansion gas topermeate into the cells of the thermoplastic resin. Preferably, thepressure is released from the foam before the temperature is allowed todrop, although the order in which these variables are allowed to drop isnot critical to achieving the density decreases.

The first step discussed above involves, effectively, a pretreatment ofthe primarily foamed thermoplastic resin material, prior to conductingthe secondary blowing. The elevated temperature during the pretreatmentacts in two ways to increase the size of the cells of the material. Thetemperature increase causes an expansion of the primary blowing gaswhich is already in the cells. The elevated temperature also affects thematerial walls, softening the material and allowing it to expand moreeasily. Thus, the temperature is chosen so that the primarily foamedmaterial is soft enough to further expand, yet is not so soft that theexisting cell structure collapses or the cell walls tear. Typicaltemperatures used during this pretreatment step range from about 80° C.to about 120° C. for poly(ethylene terephthalate), and from about 25° C.to about 60° C. for polyethylene. Moreover, the temperature is selectedbased on the particular polymer being treated. The temperature should bebetween the glass transition temperature and the melting temperature ofthe polymer comprising the semicrystalline resin material, and betweenthe softening temperature and melting temperature of the polymer if itis amorphous.

The temperature, although selected with the above considerations inmind, is also dependent on the amount of vacuum pulled and the type ofpolymer which comprises the material. Thus, there is an interdependencebetween the vacuum pulled and the temperature of the system during whichthe subatmospheric pressure is applied. The subatmospheric pressure mustbe adequate to cause the expansion of the cell, in conjunction with thetemperature elevation. The pressure may be reduced to any point lessthan atmospheric. The pressure can be reduced as far as technicallypossible. Pressures can, thus, go as low as 1 Pa. The pressure isgenerally reduced to at least 5 Pa below atmospheric, and preferably toat least 100 kPa below atmospheric. Thus, the pressure reduction willrange from 0 gauge down to about −100 kPa gauge, and preferably it isfrom about −5 Pa gauge to about −96 kPa gauge. The 0 gauge level isatmospheric pressure and can be achieved when the foam is cool and onlyneeds to be heated to be brought into the correct temperature range fornecessary expansion.

In some cases, depending on the polymer, lowering the pressure duringthe first step is not even necessary. Where only a small amount ofincrease in volume is desired in order to obtain the necessarystretching, merely increasing the temperature can work if thetemperature was not already in the desired range. In an exemplaryembodiment, both a temperature increase and a pressure decrease areeffected on the material. This combination provides a substantialdensity decrease.

After the first step is complete, the material often appears deflated.This is because the cells have been effectively stretched during thefirst step. When the pressure on the material is returned to atmosphericpressure, the material appears crushed. At this point, the material isready to be secondarily blown.

In this second step, the material is subjected to an increasedtemperature and pressure while being exposed to the secondary blowingagent. Any superatmospheric pressure can be used. The pressure can gofrom as little as 1 Pa up to the technical limit of pressure vesselsused—up to about 10,000 atmospheres. At the lower levels of 1-10 Pa,however, the step will proceed undesirably slowly. Therefore, thepressure should be up to a level in the range of from about 500 kPa toabout 10,000 kPa. A range from as little as 100 Pa over atmosphericpressure to about 1000 kPa is a preferred pressure range for thesuperatmospheric pressure of the second step.

The secondary blowing agent, then, under pressure, permeates the cellwalls and joins any remaining primary blowing agent inside each cell.Generally, the higher the pressure and temperature, the faster thesecondary expanding agent will impregnate the cells. Permeationmodifiers such as glycerol monosterate or fatty acids will also have aneffect on the permeation rate. Generally, any appropriate gas may beused as the secondary blowing agent. Typical gases include carbondioxide, air, nitrogen, argon, fluorocarbons, hydrochlorofluorocarbons,and hydrocarbon gases. These and like gases can be used alone or incombination. When the higher pressure is released, the cells reinflateand expand beyond their original volume. The lower density and higherflexibility is thereby achieved, and the material is dimensionallystable. It can now be further processed by molding, forming, etc. ifdesired.

The invention can be practiced in several different manufacturingtechniques, including both batch and continuous operations. FIG. 1illustrates, schematically, an example of a continuous operation. Aprimarily foamed resin sheet 100 is transported to a chamber 110 whereinit is exposed to reduced pressure and increased temperature by passingsheet 100 around rollers 120 configured so as to give sheet 100 adeviated route within chamber 110. The length of the deviated routearound rollers 120, the number of rollers 120, and the rate of transportwill provide the appropriate residence time for sheet 100. The residencetime requirements are determined based on the amount of expansiondesired prior to the secondary blowing, as discussed above. Sheet 100can then be transported, optionally through a second series of rollers(not shown), to a second chamber 130 wherein secondary blowing isallowed to occur. When the foam being used is taken directly from anextruder, it might already be at the desired temperature. In such acase, the pressure alone needs to be adjusted.

The process may, alternatively, be carried out in a batch operation. Insuch a case, the primarily foamed material to be treated is placed intoa first chamber wherein pressure is reduced and temperature isincreased. After expansion is allowed to occur, the material is removedand placed into a second chamber where the secondary blowing iseffected. Alternatively, the same chamber may be used for both steps,wherein the temperature and pressure conditions are manipulated atdifferent times to effect both steps during the overall residence time.

Suitable resins include any thermoplastic material. Preferred materialsinclude polystyrene polymers (amorphous) such ascopoly(styrene-acrylonitrile), olefinic polymers (semicrystalline) suchas polyethylene, polycarbonate polymers, and polyester polymers(semicrystalline) such as poly(ethylene terephthalate). Thethermoplastic foams of the present invention, although made from thesepolymers, may also include additives to enhance the overall desirabilityof the foam. One such additive would add fire retardant characteristicsto the foam. Other additives could also be added, such as to affect thecolor of the final product.

The primarily foamed material which is the starting point for thepresent invention may be of any size or shape. Sheet material ispreferred if a continuous operation is going to be used. In addition,sheet material can later be formed to the desired shape after densityreduction has been achieved. The primarily foamed material issubstantially amorphous in nature at the time of the vacuum expansion.Materials that are predominately crystalline at the time of vacuumexpansion do not sufficiently achieve the low densities embodied in thepresent invention. In an exemplary embodiment, the primarily foamedmaterial is substantially closed cell so that the cells can bepressurized by the secondary expanding agent. The primarily foamedmaterial in this embodiment also has a substantial portion of theprimary foaming agent retained in the cells.

The thermoplastic polymer foams of this invention are particularly wellsuited for use as thermal insulators. The thermal-conductivity ofthermoplastic foams (λ) is generally comprised of thethermal-conductivity of the solid phase (λ_(s)), the gas phase (λ_(g)),the convective (λ_(c)), and the radiative (λ_(r)) components:

λ=λ_(s)+λ_(g)+λ_(c)+λ_(r)

Conduction through the thermoplastic foam material itself (the cell wallmaterial) amounts to about 25% of the total energy transfer, andconduction through the gas within the cells amounts to about 50% of thetotal energy transfer. The effect of natural convection within the cellwalls is a function of the cell dimension. The overall effect of cellgas convection energy transfer on total insulation characteristics is,however, considered to be small for all but very low density foamshaving very large cell size. The final cell size of the presentinvention will depend on the original cell size of the primarily blownmaterial. This can be varied in a large range from very small to verylarge.

In order to reduce the thermal conductivity of the polymeric foam it isnecessary to minimize all of the above factors. Reducing the foamdensity, therefore, helps to lower the thermal conductivity. Additionalbenefits to using the low density foams of the present invention areeconomic. Because there is less material cost, the insulations arecheaper to manufacture, yet perform better than insulators made withfoams of higher density. Transport costs are decreased because ofdiminished weight. Construction costs are decreased because theinsulation is easier to handle. Insulations made with the presentinvention would include all types, especially tube or sheet insulationsfor pipe and ductwork.

The following examples provide detailed information on several foamsmade according to the present invention.

EXAMPLES Example 1

A sample of a primarily foamed poly(ethylene terephthalate) material wasplaced into a vessel at 116° C. and −100 kPa for 5 minutes. The densityof the material before being placed into the vessel was approximately0.16 grams per cubic centimeter. The vacuum was released and the foamsample was placed into a temperature controlled, pressure vesselcontaining carbon dioxide at 180° C. and 689 kPa for 4 hours. Thepressure was then released from the vessel and the carbon dioxide wasallowed to escape. The density of the sample after the secondaryexpansion, measured by water immersion, was approximately 0.0080grams/cc. The secondarily expanded sample was also much more flexiblethan it was in its primarily expanded state.

The nearly same experiment was also run without the vacuum step. Asimilar sample of poly(ethylene terephthalate) material having a densityof 0.16 grams per cubic centimeter was placed into a pressure vesselcontaining carbon dioxide at 180° C. and 689 kPa for 4 hours. The finaldensity of the sample was 0.15 grams/cc.

Example 2

A sample of a primarily foamed, low density poly(ethylene) tube wasplaced into a vessel at 35° C. and −68 kPa for 3 minutes. The density ofthe material before being placed into the vessel was approximately 0.024grams per cubic centimeter. The vacuum was released and the foamed tubewas placed into a temperature controlled, pressure vessel containingcarbon dioxide at 32° C. and 689 kPa for 14 hours. The pressure was thenreleased from the vessel and the carbon dioxide was allowed to escape.The density of the sample after the secondary expansion, measured bywater immersion, was approximately 0.019 grams/cc.

The nearly same experiment was also run without the vacuum step. Asimilar sample of low density poly(ethylene) tube having a density of0.024 grams per cubic centimeter was placed into a pressure vesselcontaining carbon dioxide at 32° C. and 689 kPa for 14 hours. The finaldensity of the sample was 0.024 grams/cc.

Example 3

A sample of a primarily foamed poly(ethylene terephthalate) material wasplaced into a vessel at 116° C. and −80 kPa for 5 minutes. The densityof the material before being placed into the vessel was approximately0.16 grams per cubic centimeter. The vacuum was released and the foamsample was placed into a temperature controlled, pressure vesselcontaining air at 180° C. and 838 kPa for 1 hour. The pressure was thenreleased from the vessel and the air was allowed to escape. The densityof the sample after the secondary expansion, measured by waterimmersion, was approximately 0.016 grams/cc. The secondarily expandedsample was also much more flexible than it was in its primarily expandedstate.

The nearly same experiment was also run without the vacuum step. Asimilar sample of poly(ethylene terephthalate) material having a densityof 0.16 grams per cubic centimeter was placed into a pressure vesselcontaining air at 180° C. and 838 kPa for 1 hour. The final density ofthe sample was 0.21 grams/cc.

Example 4

A sample of a primarily foamed poly(ethylene terephthalate) material wasplaced into a vessel at 80° C. and −100 kPa for 5 minutes. The densityof the material before being placed into the vessel was approximately0.16 grams per cubic centimeter. The vacuum was released and the foamsample was placed into a temperature controlled, pressure vesselcontaining carbon dioxide at 180° C. and 138 kPa for 4 hours. Thepressure was then released from the vessel and the air was allowed toescape. The density of the sample after the secondary expansion,measured by water immersion, was approximately 0.048 grams/cc. Thesecondarily expand sample was also much more flexible than it was in itsprimarily expanded state.

The nearly same experiment was also run without the vacuum step. Asimilar sample of poly(ethylene terephthalate) material having a densityof 0.16 grams per cubic centimeter was placed into a pressure vesselcontaining carbon dioxide at 180° C. 138 kPa for 4 hours. The finaldensity of the sample was 0.13 grams/cc.

The following table summarizes the results of the above examples and thepercent change in density in each case.

TABLE Final δ without Final δ with Starting δ Vacuum Step Vacuum StepExample (g/cc) (g/cc) (% decrease) (g/cc) (% decrease) 1 0.16 0.15(6.3%) 0.0080 (95.0%) 2 0.024 0.024 (0%) 0.019 (20.8%) 3 0.16 0.21(−31.3%) 0.016 (90%) 4 0.16 0.13 (18.8%) 0.048 (70.0%)

As seen from the above, in each case where the material was processed inaccordance with the present invention, large decreases in density areseen and range from approximately 20% to 95%. In cases where thematerial was exposed only to secondary blowing, small decreases wereseen, ranging from no change to approximately 19%. In one case, anincrease in density was seen. These data show the effectiveness ofpretreating the material with the vacuum step.

Although illustrated and described herein with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the spirit of the invention.

What is claimed:
 1. A thermoplastic polymer foam having a meltingtemperature and being either an amorphous thermoplastic polymer foamwith a softening temperature or a semicrystalline thermoplastic polymerfoam with a glass transition temperature, said thermoplastic foamfurther having a lower density and increased flexibility after beingsubjected to a process comprising the steps of: (a) decreasing thepressure on a primarily foamed thermoplastic polymer foam which had aninitial density and which was under an initial pressure to asubatmospheric pressure, and adjusting the temperature of said primarilyfoamed thermoplastic polymer foam to be in a range of less than saidmelting temperature of said thermoplastic polymer foam and greater thansaid softening temperature when said thermoplastic polymer foam isamorphous, or greater than said glass transition temperature when saidthermoplastic polymer foam is semicrystalline, whereby saidthermoplastic foam expands; (b) then exposing said thermoplastic polymerfoam to a superatmospheric pressure and a secondary expansion gas for asufficient amount of time to cause said gas to permeate into saidthermoplastic polymer foam; and (c) then releasing said superatmosphericpressure acting on said thermoplastic polymer foam whereby saidthermoplastic polymer foam expands.
 2. The thermoplastic polymer foam ofclaim 1 wherein during step (c) said thermoplastic foam is in the samerange as during step (b).
 3. The thermoplastic polymer foam of claim 1which has a density in the range of from 0.008 to 0.02 grams/cubiccentimeter after step (c).
 4. The thermoplastic polymer foam of claim 1which is comprised of poly(ethylene terephthalate).
 5. The thermoplasticpolymer foam of claim 1 which is comprised of poly(ethylene).
 6. Thethermoplastic polymer foam of claim 1 which is comprised of athermoplastic polymer which is one of a polystyrene and a polyolefin. 7.The thermoplastic polymer foam of claim 1 wherein said thermoplasticpolymer foam has a density in the range of from about 15% to about 96%lower than the initial density of the primarily foamed thermoplasticpolymer foam.
 8. A thermoplastic polymer foam having a meltingtemperature and being either an amorphous thermoplastic polymer foamwith a softening temperature or a semicrystalline thermoplastic polymerfoam with a glass transition temperature, said thermoplastic polymerfoam having a lower density and increased flexibility after beingsubjected to a process comprising the steps of: (a) heating a primarilyfoamed thermoplastic polymer with an initial density to a temperaturesufficient to cause said thermoplastic polymer foam to expand; (b)exposing said thermoplastic polymer foam to a superatmospheric pressureand a secondary blowing gas for a sufficient amount of time to causesaid gas to permeate into said thermoplastic polymer foam; and (c)releasing said superatmospheric pressure on said thermoplastic polymerfoam whereby said thermoplastic polymer foam expands.
 9. Thethermoplastic polymer foam of claim 8 wherein during step (c) thetemperature of said thermoplastic polymer foam is in a range of lessthan said melting temperature of said thermoplastic polymer foam andgreater than said softening temperature if said thermoplastic polymerfoam is amorphous, or greater than said glass transition temperature ifsaid thermoplastic polymer foam is semicrystalline.
 10. Thethermoplastic polymer foam of claim 9 wherein during step (c) saidthermoplastic foam is under said temperature of step (b).
 11. Thethermoplastic polymer foam of claim 8 which has a density in the rangeof from 0.008 to 0.02 grams/cubic centimeter after step (c).
 12. Thethermoplastic polymer foam of claim 8 which is comprised ofpoly(ethylene terephthalate).
 13. The thermoplastic polymer foam ofclaim 8 which is comprised of poly(ethylene).
 14. The thermoplasticpolymer foam of claim 8 which is comprised of a thermoplastic polymerwhich is one of a polystyrene and a polyolefin.
 15. The thermoplasticpolymer foam of claim 8 wherein said thermoplastic polymer foam has adensity in the range of from about 15% to about 96% lower than saidinitial density of said primarily foamed thermoplastic polymer foam.